1. Field of the Invention
The subject matter disclosed generally relates to the field of semiconductor image sensors.
2. Background Information
Photographic equipment such as digital cameras and digital camcorders contain electronic image sensors that capture light for processing into a still or video image, respectively. There are two primary types of electronic image sensors, charge coupled devices (CCDs) and complimentary metal oxide semiconductor (CMOS) sensors. CCD image sensors have relatively high signal to noise ratios (SNR) that provide quality images. Additionally, CCDs can be fabricated to have pixel arrays that are relatively small while conforming with most camera and video resolution requirements. A pixel is the smallest discrete element of an image. For these reasons, CCDs are used in most commercially available cameras and camcorders.
CMOS sensors are faster and consume less power than CCD devices. Additionally, CMOS fabrication processes are used to make many types of integrated circuits. Consequently, there is a greater abundance of manufacturing capacity for CMOS sensors than CCD sensors.
To date there has not been developed a CMOS sensor that has the same SNR and pixel pitch requirements as commercially available CCD sensors. Pixel pitch is the space between the centers of adjacent pixels. It would be desirable to provide a CMOS sensor that has relatively high SNR while providing a commercially acceptable pixel pitch.
The image sensor is typically connected to an external processor and external memory. The external memory stores data from the image sensor. The processor processes the stored data. The data includes one or more images generated by exposing the pixels for a predetermined time interval. The exposure time of the pixels is typically controlled by an internal clock(s) of the image sensor.
The exposure time of a picture frame is established by a word written into an exposure time register. Changing the exposure time requires writing new data into the register and then reading the data. In video and fast successive still photo shots this technique may create confusion regarding the exposure time of incoming pixel data, thereby creating instability in the system. It would be desirable to provide processor control of the exposure time of the pixels that improves stability and does not require an undesirable number of pins and signals.
Camera or camcorder products typically have an auto-focus function. To increase the speed of an auto-focus cycle the camera may be designed to process only a “window” of the pixel array. The auto-focus routine may require the window to move around the pixel array of the image sensor. It would be desirable to provide processor control of the window data in a manner that minimizes the pin count and number of signals required for the image sensor.